Liquid crystal display (LCD) is in great demand because of the spread of mobile computer and the expansion of cellular phone market triggered by multi-functional cell-phone. With the progress of high density packaging technique, LCD realizes slimness and miniaturization, and at the same time the LCD screen becomes bigger and wider.
It was doubtful at the beginning that large screen over 20 inches of LCD would be realized, but in fact the screen becomes larger and larger very fast. Up to now, major LCD makers have tried to produce small panel less than 20 inches, but they are going to expand rapidly their production range over 20 inches by taking advantage of high technology.
So, the glass used for the production of LCD screen is becoming larger. When there is something inferior in a product in early lamination stage, optical film has to be removed and liquid crystal cells are washed to be reused. At this time, a conventional pressure sensitive adhesive with high peel strength makes the peeling of optical film difficult and further makes high priced LCD cells destroyed.
Thus, efforts have been made to develop a high functional pressure sensitive adhesive which is fitted for the production of larger LCD. As a reference, Japanese Patent No. 3022993 describes acrylic pressure sensitive adhesive composition containing silane compound having epoxy available for the production of polarizer having excellent durability under high temperature and high humidity.
However, when silane compound having epoxy is applied to a pressure sensitive adhesive containing carboxyl (—COOH), removability is so bad to destroy liquid crystal cell because of its high peel strength. Thus, it is required a pressure sensitive adhesive composition showing not only excellent removability in the early adhesive stage when it is applied to a pressure sensitive adhesive containing any kind of functional group, for example carboxyl (—COOH) or hydroxyl (—OH), but also excellent durability in the late stage which is increased under high temperature and high humidity.
Japanese Patent Publication No. 8-104855 describes a pressure sensitive adhesive composition prepared by binding acrylic polymer with silane compound containing β-keto ester and alkoxy, which shows good peel strength to adhere polarizer on the surface of substrate and at the same time enables peeling of the polarizer out of the surface of substrate without damage of the substrate or leaving the adhesive.
The above Patent Publication describes that the addition of silane compound maintains the proper peel strength for the adhesion of polarizer onto substrate, keeps the peel strength at the level not to be over-strengthened, even by heat, and enables removal of polarizer without damage of liquid crystal device.
In order to be fitted for the production of larger LCD screen, a pressure sensitive adhesive should have low initial peel strength for possible removal, but have excellent peel strength later under high temperature or high temperature and high humidity, otherwise durability is in doubt, accompanying bubbles or edge lifting.
It is also described in Japanese Patent Publication No. 8-199144 that acrylic resin produced from polymerization of acrylic monomer in the presence of silane compound is mixed with crosslinking agent to provide a pressure sensitive adhesive composition having cohesive force and peel strength with less variation by high temperature or high temperature and high humidity and having excellent peel strength even on a curved surface.
It further describes that by the addition of silane compound, the peel strength can be properly maintained to hold polarizer on the surface of substrate, which is not over-strengthened, though, to make peeling of polarizer possible without damage of liquid crystal device.
It is better for a pressure sensitive adhesive composition to have excellent durability not carrying bubbles or edge lifting than to have cohesion and peel strength with less variation under high temperature or high temperature and high humidity. That is, it is preferred that the peel strength in the early stage of adhesion onto glass is just as good as removability is possible, but it gets higher as time goes by to keep strong and stable peel strength since there is no need of peeling in the late stage.